In all recent mammals the internal carotid artery at the level of the auditory capsule is derived from the embryonic dorsal aorta. In adults of various groups, the artery lies in positions which are classified as medial or promontory. It is shown that these character states arise by a process of differential growth affecting the relative positions of the dorsal aorta and the cochlear promontory. There is no evidence for separate vessels in the embryo which could give rise to non-homologous internal carotid arteries in the two positions. It follows that the concept of a primitive mammalian morphotype with both a medial and a promontory internal carotid artery in the same individual presents a formidable embryological problem. It is recommended that, in the absence of supporting evidence, the widespread current taxonomic and palaeontological use of this hypothetical morphotype should be regarded with suspicion.